1. Field of the Invention
The invention relates generally to the field of electrical systems, and more particularly, the invention relates to methods and systems for incipient fault detection in traction vehicles, such as locomotives or transit vehicles.
2. Brief Description of the Prior Art
Locomotives and transit vehicles, as well as other large traction vehicles used for heavy haul applications, commonly use many high power electrical components such as alternators, dynamic braking grids, traction motors, blowers, and the like. These components fail due to various reasons, one of them being grounds typically caused by insulation degradation. For example, locomotives operate in an environment subject to constant freezing and thawing which degrades the insulation system by producing cracks in the electrical insulation. The traction drive system of a locomotive has many insulated windings, and excessive leakage current may develop between the drive system and the frame of the locomotive over time due to such things as aging, moisture, abrasions, dirt and the like. This is especially true for traction motors since moisture often gets into these components because of their proximity to the rail and the accessibility of environmental conditions. Failures due to excessive electrical leakage currents in an electrical system of locomotives are a leading cause of system shutdowns and mission failures.
Leakage current detectors are standard on many kinds of electrical equipment to protect the equipment from damage or to protect personnel from injury, and there is extensive industrial background on leakage current monitoring by traditional methods in ac systems. Ground faults may occur as a result of a fault in any of a number of different system components. In the context of a locomotive, such components include the propulsion drive system, batteries, and auxiliary equipment. Within the propulsion drive system, ground faults can occur in one or several components which include alternator, rectifier, cabling, traction motor, dynamic brake resistor, and blower motor.
However, many ground are transitory in nature. Often when a fault occurs, the portion of the electrical system is deactivated, and the locomotive is scheduled for repairs. However, once the locomotive is shopped for repairs, the system no longer exhibits abnormal grounds and the repairmen cannot identify the source of the fault. This is often caused because the excessive leakage current is caused by moisture in the electrical components. By the time the locomotive is shopped, the moisture has dried out, thus eliminating the high leakage currents. The amount of moisture that is able to penetrate the insulation system and result in high leakage currents often depends in part on the condition of the insulation system. A healthy system experiences relatively small change in leakage current as a result of changing moisture conditions, whereas a system with degraded insulation may experience large changes in leakage current that is moisture dependent. Furthermore, leakage current depends on the voltage level of the system, and conventional methods generally only provide sensitive protection when the voltage is at its highest level.
It is desirable to have early warning of insulation leakage development, in motors and drives for example, so that action may be taken before there is a failure. Ground faults resulting from insulation defects and deterioration generally develop over time, on the order of days, before they cause any damage, i.e., before the leakage current reaches a critical level. Commonly assigned U.S. Pat. No. 6,421,618 entitled Incipient Leakage Current Fault Detection Apparatus and Method issued Jul. 16, 2002 to Kliman et al., which is hereby incorporated by reference in its entirety, describes methods of measuring leakage current and predicting incipient faults in an electrical system. However the prior art is deficient in monitoring leakage current that may be moisture related and methods of operating the locomotive in such a way to minimize mission failures.
It is therefore desirable to improve availability of leakage detection in electrical systems to enable incipient fault detection, and, in some cases, to continue operation of the electrical system if the leakage current is moisture related.
It is further desirable to reduce locomotive mission failures and reduce maintenance costs.
Briefly, according to one embodiment, the invention is a method for operating an electrical system including an electrical machine to predict an incipient fault in the electrical machine. The method includes generating a neutral-to-ground leakage signal representative of current between a neutral point of the machine and an electrical ground and storing the ground leakage signal in a memory. The method also includes determining temperature criteria of the electrical system and operating the electrical system at a reduced power condition if the leakage signal exceeds a selected threshold and if the selected temperature criteria is not met. The method also includes operating the electrical system at normal power conditions if the leakage signal is below the selected threshold. In a further embodiment, the method includes analyzing the stored ground leakage signals to predict an incipient fault in the electrical system. The method also can include terminating operation of the electrical system if the leakage signal exceeds the selected threshold and the temperature criteria are met.
According to another embodiment of the invention, the invention includes a system for predicting an incipient fault in an electrical machine of a locomotive. The system includes a leakage current detector capable of determining a neutral-to-ground leakage signal representative of current between a neutral point of the machine and an electrical ground and a memory storing the ground leakage signal. The system also includes a sensor for monitoring a temperature criteria of the electrical machine, and a processor responsive to the detector and sensor for preventing the operation of the locomotive at normal power conditions when the leakage signal exceeds a selected threshold and the temperature criteria is met.